Gardening Answers Knowledgebase

I noticed at the Flower and Garden Show that King County Master Gardeners were giving away samples of biosolids for use in the garden. Is sewage sludge really safe? Is it acceptable if you garden organically?

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The Environmental Protection Agency defines biosolids as treated sewage sludge. In January 2009, the EPA surveyed samples. Here is a brief summary of some of the findings:

The four anions were found in every sample.

27 metals were found in virtually every sample, with one metal (antimony) found in no less than 72 samples.

Of the six semivolatile organics and polycyclic aromatic hydrocarbons, four were found in at least 72 samples, one was found in 63 samples, and one was found in 39 samples.

Of the 72 pharmaceuticals, three (i.e., cyprofloxacin, diphenhydramine, and triclocarban) were found in all 84 samples and nine were found in at least 80 of the samples. However, 15 pharmaceuticals were not found in any sample and 29 were found in fewer than three samples.

Of the 25 steroids and hormones, three steroids (i.e., campesterol, cholestanol, and coprostanol) were found in all 84 samples and six steroids were found in at least 80 of the samples. One hormone (i.e., 17a-ethynyl estradiol) was not found in any sample and five hormones were found in fewer than six samples.

All of the flame retardants except one (BDE-138) were essentially found in every sample; BDE-138 was found in 54 out of 84 samples.

Here is King County's own information on biosolids recycling. Since the EPA is currently evaluating the results of their own survey, there may be revisions to previous notions that the concentration of toxins in biosolids was so low as to be inconsequential. In fact, the Center for Food Safety has petitioned the city of San Francisco to stop distributing biosolids at "compost giveaway events." Their website has additional information about potential risks associated with using sewage sludge.

The short answer is that there is some uncertainty about the safety of using biosolids in the garden, and if you are attempting to garden organically, it may be best to avoid using them.